Ashley Nellis, one of the authors of the report, argued that the United States has a “very strange perception of justice and of youth who misbehave.”

“Unlike other countries, we fail to acknowledge their youthfulness and often other factors that contribute to their crimes,” like poverty and childhood abuse, she said.

According to the report, life sentences for all offenders are on the rise across the country, and the population of people serving life without the possibility of parole is growing especially fast, with an increase from 40,174 in 2008 to 49,081 to 2012.

The concept of a life sentence without parole originated in the United Kingdom in the 1950s and continues to be practiced almost exclusively by former British colonies, according to Alison Parker, the U.S. director for the advocacy group Human Rights Watch. In the U.S., it became popular in the 1980s and 1990s, as politicians vied to portray themselves as “tough on crime.”

“Many Latin American countries, countries in Eastern Europe and in Asia don’t have the sentence as a possibility in any part of their penal code,” said Parker. “They may have death sentences, but they don’t have the sentence that you can die in prison.”

Among those former British colonies that do sentence people to life without parole, the U.S. stands apart.

The U.S. employs the practice far more commonly than any other country, and the U.S. has declined to ratify the 1989 U.N. Convention on the Rights of the Child, which bans life without parole for children. Somalia is the only other country that has failed to sign that treaty.

The Sentencing Project report cites the case of Sara Kruzan, who was forced into prostitution at the age of 13 and killed her pimp three years later, in 1994. Although the California Youth Authority recommended that the state try Kruzan as a juvenile, the prosecutor and judge maintained that she was competent to stand trial in adult court. Kruzan was sentenced to life without parole in 1995.

This week, Schwarzenegger’s successor, Gov. Jerry Brown (D), signed a bill that seeks to expand parole opportunities for all state prisoners convicted of crimes as children.

The California legislation, SB 260, is among a growing number of indications that America’s attitude toward juvenile crimes may be changing.

In recent years, the U.S. Supreme Court has issued two decisions limiting the use of life sentences without parole for those whose crimes were committed before age 18.

In Graham v. Florida in 2010, the court ruled that juveniles could no longer be given life without parole unless they had committed homicide. Last year, in Miller v. Alabama, the court ruled that states could no longer imprison juveniles under laws that impose life without parole as a mandatory penalty for homicide. About 2,000 prisoners who are currently serving life without parole for crimes before age 18 were sentenced under those statutes.

Experts say the Supreme Court may soon hear a case about whether Miller applies retroactively -- that is, not just to people who commit crimes in the future, but also to those who are already in prison.

“The court is making it very difficult for the states to impose this sentence on kids,” said Parker. “I think it’s the step in the right direction in recognizing the profound differences between children and adults.”

“Of course, children are capable of committing very serious and violent offenses that can cause tremendous suffering,” she continued. “But children are also uniquely capable of growth and change, and a sentence that gives them no opportunity to show their capacity to change is a sentence that denies the differences between children and adults.”

Here Are All The Countries Where Children Are Sentenced To Die In Prison

U.S. Capitol Photos

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The sun rises behind the U.S. Capitol Dome early in the morning before the ceremonial swearing-in of President Barack Obama during the 57th Presidential Inauguration in Washington, Monday, Jan. 21, 2013. (AP Photo/Paul Sancya)